The invention is directed to a process for the continuous mechanical shaping of particulate materials and to a device for carrying out the process.
Mechanical shaping of materials refers to the pressing of fine, powdery, dustlike products of wide-ranging grain size or of mixtures of such products to form coarse molded articles in the shape of briquettes or as continuous ribbon-shaped pressed articles, also called shells or scabs, and to the simultaneous crushing of such articles to form free-flowing granules.
Roller presses of various constructions are known. They have two oppositely running rolls between which the powderlike products are pressed to form briquettes or scabs. If the desired shape of the end product is briquettes, the latter are separated from the fine portion and used as such. If free-flowing grainy granules are desired, e.g. as required in the pharmaceutical industry for charging tablet compressing mechanisms, the scabs are pre-crushed on different units connected subsequent to the roller press, such as so-called thumb breakers. In an additional apparatus, e.g. a hammer mill or a friction mill with rotating or oscillating rotary movements, the material passes through a perforated plate or other sieve insert and is reduced in this manner to a grain size with a fixed maximum.
The fine portions occurring in this process must be separated from the product grain in an additional work step by means of a sieve device. A return of these fine portions to the feed end of the roller press is undesirable when processing mixtures of different components and quantity proportions, particularly in the pharmaceutical industry. There is no longer a guarantee that the composition of the final product will be free of defects.
The known roller presses have two fundamental different arrangements with respect to the pressing rolls:
1. The rolls are arranged in a vertical axis relative to one another, charging is effected by a stuffing device situated in the horizontal axis, usually in the form of screw conveyors. PA0 2. The rolls are arranged next to one another on the vertical axis, the stuffing device lies in a vertical axis relative to the latter, likewise in the form of a screw conveyor or as a feed hopper which enables a direct inlet of the feed material to the pressing zone of the roller press due to static superposition.
In all of the known variants, the scab ribbons must be reduced in size at the roller press subsequently by one or more additional grinding and sieve devices. These apparatuses can only be disassembled at great cost for cleaning, which makes them substantially difficult to use.
The arrangement of the pressing rolls one above the other has the disadvantage that a considerably poorer distribution of the powder along the entire width of the pressing roll must be taken into account due to the omission of the charging due to gravitational force. As a result, sufficient powder is fed to and compressed in the center of the rolls, but the peripheries of the rolls are usually charged much less. Practically no scab can form in these peripheral areas. The fine material together with the fine portion which results from the reduction in size must be sieved out and is considered as waste in many cases.
Although arranging the pressing rolls adjacent to one another has the advantage that a better charging is ensured along the entire width of the roll in the case of free flowing material, it has the disadvantage that the force of gravity alone causes the fine portions to "shoot through". This is particularly true in constructions of roller presses which work with a fixed roll gap. Large amounts flow through the gap into the subsequent machines without being compressed just when starting the press before a sufficiently high resistance is built up between the pressing rolls. As soon as the fed product quantity changes only slightly as the forward pressure in the proportioning container decreases due to the decreasing contents, the degree of compression and accordingly the hardness of the briquette or scabs also changes. After the size reduction and further processing, e.g. into tablets, the hardness of the tablets and accordingly the release of the active ingredient of the medication also changes.
All known roller press constructions have another substantial disadvantage which is an obstacle to their use in the pharmaceutical and foodstuff industries and in other applications.
The pressing rolls of these roller presses are supported at both sides because of their construction and can only be disassembled and cleaned at immense cost and by trained mechanics.